This invention relates generally to radiation imaging devices and in particular to proximity measuring systems for determining the position of components of the radiation imaging system with respect to the subject of examination.
Medical radiation imagers, such as x-ray machines, must be accurately positioned close to the patient to provide the desired imaging information and such that components of the assembly do not physically collide with the patient. On some types of imaging equipment, such as computer tomography (CT) imagers or the like, a radiation detector, such as an x-ray image intensifier tube is positioned on a movable gantry arm opposite to another arm on which the x-ray source is disposed; the opposed arms can be swung 360.degree. around a part of a patient's body, such as the head. It is desirable that the radiation detector be positioned close to (e.g., within about 1 inch) but not touch any part of the patient as the gantry arm rotates. In such systems an operator commonly controls the position of the radiation detector by means of manual control, such as with a joystick arrangement. The end of the radiation detector assembly nearest the patient is surrounded by a donut-shaped air-bag assembly. In what is commonly called "Level I" sensing, if the air-bag assembly comes in contact with the patient, a detected change in air pressure in the air-bag causes the control system to direct cessation of movement of the system. A pressure difference of about 0.3" of water is commonly used as the threshold to prompt a Level I stop. A second level of sensing, Level II sensing, refers to a situation when an additional 0.1" change (beyond Level I) in air-bag pressure occurs, such as from slight over-travel in the gantry arm after reaching the Level I shutdown point. A Level II signal causes a complete motor shutdown and locking of the gantry arms; the Level II motor control is accomplished via hardwired relays outside of the normal computer-controlled gantry arm control circuits. After a Level II shutdown signal, the gantry arm assembly must be manually disengaged and hand-cranked away from the patient's body. This arrangement provides a dual-point failure mode in the sensing scheme. Most systems further have a contact switch disposed exterior to the air-bag that provides a further back up, such that physical contact resulting in activation of the contact micro-switches provides independent shutdown signals to the gantry arm control system.
Efficient and effective use of medical imaging equipment of this type is enhanced by operating modalities that follow the contour of the patient's body to maintain the radiation detector assembly at a desired separation from the nearest portion of the patient's body as the assembly is rotated around the body. It is desirable that no part of the radiation detector assembly and gantry arm come into contact with the patient's body at any time during the procedure, and further desirable that the control system be able to prevent contact with the patient and shutdown commands that are generated as a result of contact with the air-bag assembly disposed around the radiation detector.